Ligand amino alcohol derived

In addition, these authors have reported the synthesis of other A -sulfony-lated amino alcohols derived from camphor that have been examined as ligands... 

Sinou and coworkers evaluated a range of enantiopure amino alcohols derived from tartaric acid for the ATH reduction of prochiral ketones. Various (2R,iR)-i-amino- and (alkylamino)-l,4-bis(benzyloxy)butan-2-ol were obtained from readily available (-I-)-diethyl tartrate. These enantiopure amino alcohols have been used with Ru(p-cymene)Cl2 or Ir(l) precursors as ligands in the hydrogen transfer reduction of various aryl alkyl ketones ee-values of up to 80% have been obtained using the ruthenium complex [93]. Using (2R,3R)-3-amino-l,4-bis(benzyloxy)butan-2-ol and (2R,3R)-3-(benzylamino)-l,4-bis(benzyloxy)butan-2-ol with [lr(cod)Cl]2 as precursor, the ATH of acetophenone resulted in a maximum yield of 72%, 30% ee, 3h, 25 °C in PrOH/KOH with the former, and 88% yield, 28% ee, 120 h with the latter. 

A variety of chelate complexes of Al3+ with N and/or O donor atoms are known through stability constant data.7,8 Their formation illustrates several aspects of A1 coordination chemistry. Chelates and other multidentate ligand systems provide a means of regulating the reactivity of aluminum compounds. For example, A1 alkoxides can be converted to amino alcohol derivatives to confer water solubility and a degree of hydrolytic stability on otherwise water-sensitive materials. 

Benzoquinone 6 was reported as one of a new class of amino alcohol-derived benzoquinones tested in the palladium-catalyzed 1,4-dialkylation of 1,3 dienes. These ligands were prepared by reaction of 1 with C2-symmetric l,4-diallyloxy-2,5-benzenedicarboxylic acid chloride followed by allyl deprotection.29... 

Since the discoveries of Itsuno32 and Corey,33 remarkable advances have been made in the enantio-selective reduction of prochiral ketones using amino alcohol-derived oxazaborolidines (see Chapter 16).34 35 In most cases, these amino alcohols were obtained from chiral pool sources. Consequently, extensive synthetic manipulations were often necessary to access their unnatural antipode. Didier and co-workers were first to examine the potential of m-aminoindanol as a ligand for the asymmetric oxazaborolidine reduction of ketones.36 Several acyclic and cyclic amino alcohols were screened for the reduction of acetophenone (Scheme 17.2), and m-aminoindanol led to the highest enantioselectivity (87% ee). 

Enantioselection can be controlled much more effectively with the appropriate chiral copper, rhodium, and cobalt catalyst.The first major breakthrough in this area was achieved by copper complexes with chiral salicylaldimine ligands that were obtained from salicylaldehyde and amino alcohols derived from a-amino acids (Aratani catalysts ). With bulky diazo esters, both the diastereoselectivity (transicis ratio) and the enantioselectivity can be increased. These facts have been used, inter alia, for the diastereo- and enantioselective synthesis of chrysan-themic and permethrinic acids which are components of pyrethroid insecticides (Table 10). 0-Trimethylsilyl enols can also be cyclopropanated enantioselectively with alkyl diazoacetates in the presence of Aratani catalysts. In detailed studies,the influence of various parameters, such as metal ligands in the catalyst, catalyst concentration, solvent, and alkene structure, on the enantioselectivity has been recorded. Enantiomeric excesses of up to 88% were obtained with catalyst 7 (R = Bz = 2-MeOCgH4). 

This method of asymmetric cyclopropanation using copper catalysts which are chirally modified with salicylaldimines of optically active amines has been intensively investigated and numerous modifications of the ligands have been tested24-40-43. The use of chiral amino alcohols derived from amino acids is exceptionally successful. Thus, 2-methylpropene with ethyl diazoacetate in the presence of R-7644 (Sumitomo catalyst) gives ethyl (LS)-2,2-dimethyl-1-cyclopropanecarboxylate (2) with 92% ee, on an industrial scale24. This compound is used as a precursor of cilastm, an enzyme inhibitor. 

Fortunately, continued exploration has provided more effective reagents. The most successful utilize l,l -bi-2-naphthalenol, diamino, or amino alcohol derived modifiers. These ligands apparently effectively complex the aluminum and minimize the disproportionation problem, However, structural information is still lacking and mechanism-based predictions of the absolute configuration of the product usually cannot be made. Nevertheless, these reagents often give over 90% asymmetric induction with a variety of ketones (Table 3). 

Cobalt(III)-SALEN complexes (see Fig. 20) were found to be efficient catalysts for asymmetric cyclopropanation (184). Co(acac)2 in the presence of chiral amino alcohols (derived from camphor) has been employed as a catalyst for the enan-tioselective addition of diethylzinc to chalcone (185). Axially chiral SALEN-type ligands possessing biphenyl-core as an element of chirality are efficient ligands for the enantioselective addition of diethylzinc to aldehydes. The formation of bimetallic species forming a chiral pocket was shown (186). 

The hydride-donor class of reductants has not yet been successfully paired with enantioselective catalysts. However, a number of chiral reagents that are used in stoichiometric quantity can effect enantioselective reduction of acetophenone and other prochiral ketones. One class of reagents consists of derivatives of LiAlH4 in which some of die hydrides have been replaced by chiral ligands. Section C of Scheme 2.13 shows some examples where chiral diols or amino alcohols have been introduced. Another type of reagent represented in Scheme 2.13 is chiral trialkylborohydrides. Chiral boranes are quite readily available (see Section 4.9 in Part B) and easily converted to borohydrides. 

To obtain information about the structural requirements of a ligand capable of catalyzing the addition of dialkylzincs to aldehydes, various simple amines, alcohols and amino acid derived amino alcohols were tested as chiral catalysts (Table 27). 

Scheme 3.27 D-Cysteine-derived C2-symmetric bis-P-amino alcohol ligands for addition of ZnEt2 to benzaldehyde.

Among various amino alcohols used as ligands, derivatives of simple and easily available carbohydrates are only rarely reported and remain underestimated. In this context, a-hydroxy sulfonamides derived from D-glucosa-mine have been used by Bauer et al. for addition of ZnEt2 to aldehydes, providing the corresponding products in high enantioselectivities of up to 97% ee, as shown in Scheme 3.47. ... 

Scheme 3.50 Camphor-derived A -sulfonylated amino alcohol ligand for additions of ZnEt2 to aldehydes.

Similar reactions have been developed more recently by Jin et al. using chiral amino thioacetate ligands derived from the corresponding amino alcohols. Low catalyst loadings of only 1-2.5 mol% were sufficient to achieve excellent enantioselectivities of up to 98% ee as well as high conversions in short times (Scheme 3.61). These authors have shown that the thioacetoxy moiety of the amino thioacetates has a surprisingly beneficial effect in enhancing the asymmetric induction. 

Scheme 10.61 Borane reduction of acetophenone with D-cysteine-derived bis-P-amino alcohol ligands.

Considerable improvement was achieved with binuclear copper(II) chelates of type 195 whose ligands are derived from salicylaldehyde and an optically active amino alcohol 91>92>. 

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